1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image forming apparatus of the type including an intermediate image transfer belt.
2. Description of the Background Art
A color image forming apparatus including an intermediate image transfer body implemented as a belt or a drum belongs to a family of conventional image forming apparatuses. In the color image forming apparatus, toner images of different colors are sequentially formed on an image carrier while being sequentially transferred to the intermediate image transfer body one above the other. This image transfer will be referred to as primary image transfer. The resulting composite toner image is transferred from the intermediate image transfer belt to a sheet or recording medium. This image transfer will be referred to as secondary image transfer and is effected by a secondary image transfer roller or body and a back electrode or roller facing it. The back electrode is electrically connected to the inside surface of the intermediate image transfer body.
Some other electrodes usually adjoin the back electrode for secondary image transfer and are also electrically connected to the inside surface of the intermediate image transfer body. Such other rollers include a back electrode facing a cleaning member assigned to the intermediate image transfer body. An electric field is formed between the cleaning member and the back roller, which face each other, so that the cleaning member can collect toner left on the intermediate image transfer body after secondary image transfer.
Another electrode contacting the intermediate image transfer body is a back electrode facing a charging member configured to invert the polarity of the toner left on the intermediate image transfer body after secondary image transfer. An electric field is also formed between the charging member and the back electrode, which face each other, in order to invert the polarity of the above residual toner and then cause the toner to again deposit on an image carrier at a primary image transfer position.
Still another electrode contacting the intermediate image transfer body is a tension roller supported by the frame of the apparatus for applying tension to the image transfer body.
In this connection, Japanese Patent Laid-Open Publication No. 10-49019 discloses an image forming apparatus in which a voltage of the same polarity as toner is applied to the inside surface of an intermediate image transfer drum. By this voltage, toner left on the intermediate image transfer drum after secondary image transfer is inverted in polarity and then caused to again deposit on an image carrier at a primary image transfer station.
In the image forming apparatus of the type including the intermediate image transfer body, a voltage subject to constant-current control is applied from the back electrode for secondary image transfer to the inside surface of the above image transfer body. At the same time, the secondary image transfer roller is grounded. As a result, an electric field for secondary image transfer is formed between the intermediate image transfer body and the secondary image transfer roller. This electric field varies little even when some current flows via a recording medium or even when the resistance of the intermediate image transfer body or that of the secondary image transfer roller varies, allowing a stable image to be formed on the recording medium.
However, when any one of the electrodes adjoining the back roller for secondary image transfer roller, as stated earlier, is grounded, a current fed to the back roller for secondary image transfer leaks to the other back roller via the intermediate image transfer body. As a result, a current flowing toward the recording medium becomes short. Further, when the tension roller contacting the inside surface of the intermediate image transfer body adjoins the back roller for secondary image transfer, the current fed from the back roller for secondary image transfer to the intermediate image transfer body leaks to the frame of the apparatus via the above image transfer body, again making the current flowing toward the recording medium short.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-102737, 10-39642, 2000-19854, 2001-166614 and 2002-251076